Early and aggressive fluid resuscitation in patients suffering from shock is associated with improvement in outcome, including mortality. Multiple studies have supported this concept in a variety of clinical settings, from septic shock to high-risk surgical patients. However, a clear association also exists between cumulative fluid balance and mortality. It therefore becomes prudent to adopt a tailored approach to fluid resuscitation over empiric fluid loading. After all, a fluid bolus that does not lead to increased stroke volume is unlikely to benefit the patient, and carries all of the risk associated with volume overload.
Multiple methods, invasive or otherwise, have been proposed to predict an increase in cardiac stroke volume (referred to herein at times as SV) with a fluid bolus, or which increase in stroke volume is referred to as volume responsiveness (referred to herein at times as VR). Stroke volume (measured in ml/beat) is the amount of blood pumped out of the heart with each beat (and more specifically a cone of blood that comes through the left ventricular outflow tract (LVOT)). Stroke volume is also an important determinant of fluid status, and thus can be helpful in directing the administration of fluid boluses. While central venous pressure (CVP) has traditionally been used to assess volume status, studies have not demonstrated a reliable relationship between CVP and volume responsiveness. Pulmonary artery catheters (PAC) have also fallen out of favor due to their invasiveness and potential for serious complications. Several studies have failed to show any improvement in outcome associated with PAC use. Given its non-invasive nature, portability, and ease of use as a point-of-care test, ultrasound (US) has emerged as an attractive option to assess volume status and predict fluid responsiveness.
The most studied ultrasound measure is respiratory variation in the inferior vena cava (rvIVC). This measure is relatively easy to perform with any point-of-care ultrasound system. While multiple studies have demonstrated that rvIVC accurately predicts volume responsiveness in mechanically ventilated patients, there is conflicting evidence in spontaneously breathing patients. rvIVC measurement may be difficult or impossible to perform in many surgical patients, and particularly those presenting with abdominal distension, surgical wounds, morbid obesity, or bowel gas. Additionally, evidence suggests that in the setting of increased thoracic and abdominal pressures, IVC diameter and collapsibility indices may lose their reliability.
Given the varied and limited successes of prior known methods, which generally assess only one metric, it would be advantageous to provide more easily implemented system that allows several ultrasound metrics to be included in the measurements and predictions. This would allow healthcare providers to administer fluid only to those patients most likely to have a positive response, who have a low stroke volume to begin with and limiting application to those least likely to have a positive response (and thereby reducing the risk of volume overload in such patients). It would also be advantageous to provide systems and methods of using noninvasive ultrasound measurements to predict whether a given patient will be likely to have a positive volume response to a fluid bolus prior to its administration.